Enhanced photocatalytic hydrogen evolution over Pt supported on mesoporous TiO2 prepared by single-step sol–gel process with surfactant template

International Journal of Hydrogen Energy 31 (2006) 786 – 796 www.elsevier.com/locate/ijhydene

Enhanced photocatalytic hydrogen evolution over Pt supported on mesoporous TiO2 prepared by single-step sol–gel process with surfactant template Thammanoon Sreethawong, Susumu Yoshikawa∗ Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan Available online 1 August 2005

Abstract Nanocrystalline mesoporous TiO2 photocatalyst with narrow pore size distribution was synthesized by a surfactant-assisted templating sol–gel technique and used as a support for Pt towards photocatalytic H2 evolution. Single-step sol–gel (SSSG) method, in which Pt precursor was introduced into the completely hydrolyzed TiO2 sol prepared with a mesopore-directing surfactant template, was utilized to load the Pt cocatalyst onto the synthesized mesoporous TiO2 . In comparison, the conventional loading methods namely incipient wetness impregnation (IWI) and photochemical deposition (PCD) were also performed. The mesoporous Pt/TiO2 photocatalyst prepared by the SSSG method showed higher photocatalytic activity for H2 evolution over entire range of Pt loading content (optimum loading of 0.6 wt%) as well as broader photocatalytic activity window with respect to the amount of loaded Pt than those prepared by the other two methods and Pt/Degussa P-25 TiO2 . Hence, the SSSG method has been postulated as a beneficial alternative for synthesis of effective metal-loaded mesoporous photocatalyst. 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen evolution; Photocatalyst; Pt/TiO2 ; Surfactant template; Sol–gel process

1. Introduction The mesoporous materials with different compositions, new pore systems, and novel properties have attracted considerable attention because of their remarkably large surface areas and narrow pore size distributions, which make them ideal candidates for catalysis [1]. Since the first discovery of mesoporous silicate and aluminosilicate materials (MCM-41) with uniformly sized pores and large surface area by using amphiphilic template-directing method [2], many attempts have been devoted to broaden the surfactant templating procedure to the formation of non-silica mesoporous materials with a narrow pore size distribution and ∗ Corresponding author. Tel.: +81 774 38 3504; fax: +81 774 38 3508. E-mail addresses: [email protected] (T. Sreethawong), [email protected] (S. Yoshikawa).

controlled pore structure [3–5]. It is generally anticipated that the use of a high surface area mesoporous oxide support rather than a commercially low surface area support for noble metals or transition metals has some beneficial effects on the catalytic performance [6]. The mesoporous support would give rise to well dispersed and stable metal particles on the surface upon thermal treatments and as a consequence would also show an improved catalytic efficiency. In photocatalysis, titania (TiO2 ) is the most attractive semiconductor among transition metal oxides because of its excellent performance in photocatalytic reactions [7–9]. However, the usage of mesoporous TiO2 both itself and as a support for metal loading in this field still remains as a challenging task in both scientific and practical viewpoints. The concentration in platinum-loaded titania (Pt/TiO2 ) has rapidly increased due to its high activity in a multitude of important reactions [10–14]. Particularly, the photocatalytic H2 generation from water splitting has recently been

0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.06.015

T. Sreethawong, S. Yoshikawa / International Journal of Hydrogen Energy 31 (2006) 786 – 796

attracting fast growing interest due to clean, storable, and renewable energy development, aiming to replace exhausting fossil fuels by use of solar energy [15–17], ever since Fujishima and Honda’s report on the photoelectrochemical water splitting by TiO2 electrode [18]. Amongst several previous works, commercially available Degussa P-25 TiO2 has been prevalently utilized as a support for platinum loading through photochemical deposition method. However, in the photocatalytic perspective, the photocatalytic activity and especially the stability of platinum strongly depend on both the state and structure of the support and the specific interaction between platinum and support. The type of the support is of crucial importance to obtain highly dispersed platinum particles with good performance [19]. To our knowledge, it has been suggested a good possibility to apply the mesoporous TiO2 as a support for platinum towards the photocatalytic reaction of H2 evolution. In our previous articles, the mesoporous TiO2 photocatalyst synthesized via the surfactant-assisted templating sol–gel process of laurylamine hydrochloride (LAHC)/tetraisopropyl orthotitanate (TIPT) modified with acetylacetone (ACA) system was reported [20,21] and used as an efficient support for nickel oxide loaded by single-step sol–gel (SSSG) method for the photocatalytic H2 evolution [22]. In this work, we present a considerably high photocatalytic performance over platinum supported on such the mesoporous TiO2 . The methods of platinum loading namely SSSG, incipient wetness impregnation (IWI), and photochemical deposition (PCD) and their characterizations by TG-DTA, XRD, N2 adsorption–desorption analysis, and TEM were relatively described. The photocatalytic H2 evolution activity of the mesoporous TiO2 photocatalyst in comparison with Degussa P-25 TiO2 with various platinum contents was tested systematically.

2. Experimental 2.1. Materials Tetraisopropyl orthotitanate (TIPT, Tokyo Chemical Industry Co., Ltd.), hydrogen hexachloroplatinate(IV) hydrate (Nacalai Tesque, Inc.), laurylamine hydrochloride (LAHC, Tokyo Chemical Industry Co., Ltd), acetylacetone (ACA, Nacalai Tesque, Inc.) and methanol (Nacalai Tesque, Inc.) were used for this study. All chemicals were analytical grade and used without further purification. LAHC was used as a surfactant template behaving as a mesopore-directing agent. ACA serving as a modifying agent was applied to moderate the hydrolysis and condensation processes of titanium precursor. Degussa P-25 TiO2 (Nippon Aerosil Co., Ltd.), which is widely used in photocatalysis, was also used for comparatively examining the photocatalytic H2 evolution activity.

787

2.2. Photocatalyst synthesis SSSG made Pt-loaded mesoporous TiO2 photocatalyst was synthesized via a combined sol–gel with surfactantassisted templating mechanism in the LAHC/TIPT modified with ACA system. In typical synthesis, a specified amount of analytical grade ACA was first introduced into TIPT with the same mole. The mixed solution was then gently shaken until intimate mixing. Afterwards, 0.1 M LAHC aqueous solution of pH 4.2 was added into the ACA-modified TIPT solution, in which the molar ratio of TIPT to LAHC was tailored to a value of 4. The mixture was kept continuously stirring at 40 ◦ C for 10 h to obtain transparent yellow sol. To the aged TiO2 sol solution, a necessary amount of hydrogen hexachloroplatinate(IV) hydrate in methanol for desired Pt loading of 0.1–0.9 wt% was incorporated, and the final mixture was further aged at 40 ◦ C for 5 days to acquire homogeneous solution. Then, the gel was formed by placing the sol-containing solution into an oven kept at 80 ◦ C for a week. Subsequently, the gel was dried overnight at 80 ◦ C to eliminate the solvent, which is mainly the distilled water used in the preparation of surfactant aqueous solution. The dried sample was calcined at 500 ◦ C for 4 h to remove LAHC template and consequently produce the desired photocatalyst. For comparative studies, IWI and PCD methods were used to load Pt onto the mesoporous TiO2 support prepared by the same sol–gel method described above. For IWI method, the support was impregnated with an appropriate amount of hydrogen hexachloroplatinate(IV) hydrate aqueous solution, dried at 80 ◦ C, and finally calcined at 500 ◦ C for 4 h. For PCD method, the support was first dispersed in distilled water, ultrasonicated, and charged to the photoreactor. Subsequently, required amounts of hydrogen hexachloroplatinate(IV) hydrate aqueous solution, methanol, and distilled water were added to obtain 50 vol% aqueous methanol solution. The mixture was magnetically stirred and irradiated with a 300 W high-pressure Hg lamp for 1 h. After irradiation, the deposited TiO2 particles were recovered by filtration, washed with hot distilled water, and dried at 80 ◦ C. The Pt/P-25 was also prepared by PCD method with the same procedure. 2.3. Photocatalyst characterizations Simultaneous thermogravimetry and differential thermal analysis (TG-DTA, Shimadzu DTG-50) with a heating rate of 10 ◦ C/min in a static air atmosphere was used to study the thermal decomposition behavior of the as-prepared (dried) photocatalysts with
-Al2 O3 as the reference. X-ray diffraction (XRD) was used to identify phases present in the samples. A Rigaku RINT-2100 rotating anode XRD system generating monochromated Cu K
radiation with continuous scanning mode at rate of 2◦ /min and operating conditions of 40 kV and 40 mA was used to obtain XRD patterns. A nitrogen adsorption system

T. Sreethawong, S. Yoshikawa / International Journal of Hydrogen Energy 31 (2006) 786 – 796

-20

10

-30

5

0

0

DTA

200

(a)

300 400 500 600 Temperature / °C

700

-5 800

20

TG

-10

15

-20

10

-30

5

-40 -50 100 (b)

3. Results and discussion

15

-50 100

Weight loss / %

Photocatalytic H2 evolution reaction was performed in a closed gas-circulating system. In typical run, a specified amount of photocatalyst (0.2 g) was suspended in an aqueous methanol solution (200 ml of distilled water, 20 ml of CH3 OH) by means of magnetic stirrer within an inner irradiation reactor made of Pyrex glass. A 300 W high-pressure Hg lamp was utilized as the light source. Prior to the reaction, the mixture was deaerated by purging with Ar gas repeatedly. To avoid the heating of the solution during the courses of reaction, water was circulated through a cylindrical Pyrex jacket located around the light source. The gaseous H2 evolved was periodically collected and analyzed by an on-line gas chromatograph (Shimadzu GC-8A, Molecular sieve 5A, Argon gas), which was connected with a circulation line and equipped with thermal conductivity detector (TCD).

-10

-40

2.4. Photocatalytic activity measurement

20

TG

0

DTA

200

300 400 500 600 Temperature / °C

DTA / µVmg-1

0

DTA / µVmg-1

(BEL Japan BELSORP-18 Plus) was employed to measure adsorption–desorption isotherms at liquid nitrogen temperature of 77 K. The Brunauer–Emmett–Teller (BET) approach using adsorption data over the relative pressure ranging from 0.05 to 0.35 was utilized to determine surface area. The Barrett–Joyner–Halenda (BJH) approach was used to yield pore size distribution from desorption data. The samples were degassed at 200 ◦ C for 2 h to remove physisorbed gases prior to the measurement. The sample morphology was observed by a transmission electron microscope (TEM, JEOL JEM-200CX) operated at accelerating voltage of 200 kV.

Weight loss / %

788

700

-5 800

Fig. 1. TG-DTA curves of dried samples: (a) unloaded TiO2 and (b) 0.6 wt% Pt/TiO2 prepared by SSSG method.

3.1. Photocatalyst characterizations Fig. 1 shows the TG-DTA curves of the dried TiO2 without and with 0.6 wt% Pt loading by the SSSG method, which displayed the highest photocatalytic activity for H2 evolution as explained later. The curve patterns of the unloaded TiO2 , which was used as the support for Pt loading by the IWI and PCD methods, had no substantial difference from those of the loaded one. The comparable total weight losses measured from the TG curves were 44.91 and 43.43 wt% for unloaded and loaded photocatalysts, respectively. The DTA curves show two main exothermic peaks. The details of the position of the exothermic peaks as well as their corresponding weight loss are summarized in Table 1. The first exothermic peak was very sharp and narrow, and was attributed to the burnout of the surfactant template. The second exothermic peak was weaker and broader, and corresponded to the crystallization process of the photocatalysts [23]. The only distinct difference can be noticed from the crystallization peak that the Pt/TiO2 released larger extent of heat for crystallization than the unloaded TiO2 ,

indicating the higher thermal stability of the TiO2 support with Pt loading. In addition, since the TG curves showed that the weight loss ended at 500 ◦ C, the calcination temperature at this value, i.e. 500 ◦ C, is sufficient for complete removal of the organic surfactant molecules. The XRD patterns of the Pt/TiO2 synthesized by the three methods as well as Pt/P-25 are shown in Fig. 2. The diffractograms of the synthesized photocatalysts mainly belong to the crystalline structure of anatase TiO2 , whereas those of the loaded P-25 comprise a mixture of anatase and rutile phases with approximately 79% anatase content, calculated by Spurr and Myers’ method [24] as shown in Table 2. The presence of Pt could be observed at high loading concentration at diffraction angle of 39.8◦ indexed to (1 1 1) plane, however the peak intensity is relatively weak, presumably due to the combination of its low content and small particle size. The crystallite size of the synthesized samples estimated from line broadening of anatase (1 0 1) diffraction

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Table 1 Thermal decomposition behavior of synthesized photocatalysts from TG-DTA analysis Photocatalyst

Unloaded mesoporous TiO2 (Support) 0.6 wt% Pt/TiO2 (SSSG method)

Position of exothermic peak/◦ C

Corresponding weight loss/wt%

1st peak

2nd peak

1st peak

2nd peak

Total

334.4 332.2

438.3 428.8

34.31 31.75

10.60 11.68

44.91 43.43

(a)

(b)

(c)

(d)

Fig. 2. XRD patterns of (a) Pt/TiO2 prepared by SSSG method, (b) Pt/TiO2 prepared by IWI method, (c) Pt/TiO2 prepared by PCD method, and (d) Pt/P-25 at various Pt loadings (A: Anatase, R: Rutile).

peak using Sherrer equation [25] is shown in Table 3. The results revealed that the crystallite size of photocatalysts prepared by the three methods is in the range of 10–13 nm. Although significant difference in crystallite size was not observed, the crystallite size of the samples prepared by IWI and PCD methods is in average slightly larger than that of the SSSG samples. This stemmed from the stabilization of the TiO2 support at 500 ◦ C before the impregnation and photochemical deposition processes, respectively. For the Pt/P-25 samples, the crystallite sizes estimated from both anatase (1 0 1) and rutile (1 1 0) peaks at diffraction angle of

25.2◦ and 27.4◦ are roughly 21 and 30 nm, respectively, as included in Table 2. It is manifest that the crystallite size of the synthesized samples is as half as that of the loaded P-25 case. Fig. 3 depicts the nitrogen adsorption–desorption isotherms and pore size distributions of the unloaded TiO2 and 0.6 wt% Pt/TiO2 prepared by SSSG method. The isotherms exhibit typical type IV pattern with hysteresis loop, characteristic of mesoporous material according to the classification of IUPAC [26]. A sharp increase in adsorption volume of N2 was observed and located in the P /P0 range

790

T. Sreethawong, S. Yoshikawa / International Journal of Hydrogen Energy 31 (2006) 786 – 796

Table 2 Physical properties of Pt/P-25 Pt loading/wt%

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Anatase contenta /%

78.9 78.7 78.7 79.1 79.0 79.0 78.5 78.8 78.4 78.6

BET surface areac /m2 g−1

Crystallite sizeb /nm Anatase(1 0 1)

Rutile(1 1 0)

20.71 20.82 20.56 21.15 20.82 20.93 20.66 20.66 21.04 21.26

29.86 29.32 30.09 29.76 29.54 29.86 29.75 30.20 29.54 30.09

51.6 51.2 50.1 46.2 45.9 42.9 40.0 34.8 35.5 31.6

a Estimated

using Spurr and Myers’ method. from line broadening of diffraction peaks using Sherrer formula. cAdsorption–desorption isotherms: IUPAC type II pattern, indicating the absence of mesopore. b Estimated

Table 3 Crystallite size of synthesized photocatalysts from XRD analysis Pt loading/wt%

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 a Estimated

Crystallite sizea /nm Unloaded TiO2 (support)

SSSG Pt/TiO2

IWI Pt/TiO2

PCD Pt/TiO2

11.07 — — — — — — — — —

— 12.12 11.69 11.73 11.53 11.66 10.90 12.06 12.02 11.49

— 12.01 11.95 12.26 12.28 12.55 12.40 12.23 12.69 12.36

— 13.43 13.06 12.25 12.17 12.04 12.75 12.36 13.04 13.08

from line broadening of anatase (1 0 1) diffraction peak using Sherrer formula.

of 0.45–0.80. This sharp increase can be imputable to the capillary condensation, indicating the good homogeneity of the sample and fairly small pore size since the P /P0 position of the inflection point is related to the pore dimension. The pore size distribution obtained by BJH approach (see inset of Fig. 3(a) and (b)) is noticeably narrow, confirming good quality of the sample. In comparison, the isotherm of the 0.4 wt% Pt/P-25 displaying the best photocatalytic activity in the case of the P-25 support is also shown in Fig. 3. It can be seen that the isotherm elucidates the typical IUPAC type II pattern, revealing the absence of mesoporous structure in the commercial P-25. The textural properties of the photocatalysts prepared by the three methods are given in Table 4. The higher the Pt content was loaded, the larger the decrease in the BET surface area of the samples for all methods was observed. In the investigated range of Pt loading content, the surface area obtained from the different methods decreased in the

following order: SSSG > IWI > PCD. The mean pore diameter of the IWI and PCD photocatalysts remained almost identical to that of the TiO2 support, while the total pore volume decreased in similar trend to the surface area. It may be concluded that such the small amount of Pt loaded disturbed the mesopore volume by depositing at the outer and near the outer surface of the TiO2 aggregates, while maintaining the general pore size characteristic. However, it is surprisingly found that the mean pore diameter of the SSSG photocatalysts fairly decreased with the increased amount of Pt loading and was lower than that of the IWI and PCD ones. This implies that in case of the SSSG method, the loaded Pt may play a significant role during the TiO2 matrix formation. In sol–gel process, it is well known that the loaded metal incorporates into the matrix of support. However, in this investigated SSSG system, the Pt precursor was added after the complete hydrolysis of TIPT during the preparation. Most of Pt crystallites are, therefore, expected to

T. Sreethawong, S. Yoshikawa / International Journal of Hydrogen Energy 31 (2006) 786 – 796

150

(a)

100 50 0

0

5 10 Pore radius / nm

15

50 Adsorption Desorption 0

0.2

0.4 0.6 Relative Pressure, P/P0

0.8

100

150 100 50 0

0

5 10 Pore radius / nm

15

50 Adsorption Desorption 0

1

dv/dR / mm3nm-1g-1

100

150

Adsorbed amount / cm3(STP)g-1

dv/dR / mm3nm-1g-1

Adsorbed amount / cm3(STP)g-1

150

0

791

0

0.2

(b)

0.4 0.6 Relative Pressure, P/P0

0.8

1

Adsorbed amount / cm3(STP)g-1

400

300

200

0 (c)

Adsorption Desorption

100

0

0.2

0.4 0.6 Relative Pressure, P/P0

0.8

1

Fig. 3. N2 adsorption–desorption isotherms of (a) unloaded TiO2 , (b) 0.6 wt% Pt/TiO2 prepared by SSSG method, and (c) 0.4 wt% Pt/P-25 (Inset of (a) and (b): pore size distribution).

deposit on the surface of the TiO2 throughout the mesopore network (some of which were inevitably buried inside the TiO2 ). By considering the diameter of micellar LAHC hydrophobic part of approximately 3.34 nm, the mean pore diameter of the SSSG samples (see Table 4) seems to get in closer proximity with increasing the Pt content as compared with the other two methods, in which bare TiO2 from the same sol–gel process was used for Pt loading. This also implies that introducing Pt precursor into the completely hydrolyzed TiO2 sol by the SSSG method can bring about the thermal stability of mesoporous framework during the crystallization of the walls separating mesopores upon thermal treatment, subsequently resulting in the less loss of surface area. In case of the Pt/P-25, the BET surface area of the samples at various Pt loading amounts is given in Table 2. Apparently, it can be discernible that the surface area decreased with increasing the Pt content and was quite lower than that of the synthesized samples.

TEM images of the Pt/TiO2 prepared by the three methods (at the Pt loading content, which showed the highest photocatalytic activity for each method) are shown in Fig. 4. It is made evident that the nanocrystalline TiO2 aggregates was formed with three-dimensional disordered primary particles. The observed TiO2 particle size of 10–15 nm is in good accordance with the crystallite size estimated from XRD, elucidating that each grain can be reliably considered in average as a single crystallite. From visualization of the images, there are a few dark patches in the images, indicating high electron density, and it corresponds to the deposition of Pt nanoparticles. They are relatively and homogeneously dispersed in the case of SSSG and IWI methods rather than the PCD method, of which aggregation of Pt particles seems to more easily take place on the TiO2 surface. Average Pt particle size of the SSSG and IWI photocatalysts is approximately 1 nm, while it becomes larger and more diverse up to 5 nm due to some aggregations in the case of PCD method.

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Table 4 Textural properties of synthesized photocatalysts from N2 adsorption--desorption analysis Photocatalyst

Pt loading/wt%

BET surface area/m2 g−1

Mean pore diameter/nm

Total pore volume/cm3 g−1

Unloaded TiO2 (support)

—

100.9

6.34

0.216

SSSG Pt/TiO2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

92.2 91.6 89.5 91.2 87.9 89.0 87.1 87.0 83.3

6.34 5.34 5.06 5.34 5.34 5.06 4.82 4.82 4.36

0.189 0.184 0.182 0.182 0.178 0.162 0.161 0.154 0.144

IWI Pt/TiO2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

85.9 84.7 82.7 79.7 81.7 79.1 79.7 78.7 76.4

7.22 6.34 7.22 6.76 6.34 6.76 5.96 6.34 5.96

0.199 0.200 0.190 0.190 0.189 0.188 0.185 0.179 0.168

PCD Pt/TiO2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

84.0 71.7 70.7 65.6 62.7 60.8 55.1 57.1 52.7

5.96 6.34 6.34 6.76 6.76 6.34 6.34 6.76 6.34

0.192 0.175 0.180 0.163 0.160 0.158 0.154 0.152 0.153

3.2. Photocatalytic activity Photocatalytic activity of the Pt/TiO2 prepared by the three methods was quantitatively examined by photocatalytic H2 evolution from the suspension of the photocatalysts with various Pt loadings (0.1–0.9 wt%) in an aqueous methanol solution for 5 h irradiation using 300 W highpressure Hg lamp (The lamp emits mostly UV light.). Briefly about the mechanism of photocatalytic H2 evolution over Pt/TiO2 , the reaction is initiated by the photoexcitation of TiO2 particles, which leads to the formation of electron–hole pairs. Photogenerated conduction band electrons can be transferred to electron acceptor, H+ . With Pt on TiO2 photocatalyst, Pt can trap electrons and hydrogen can be produced on Pt by reduction reaction. Likewise, valence band holes can be filled by the electron donor, CH3 OH, resulting in the great prevention of photoinduced charge recombination [15,16]. Control experiments showed that no appreciable hydrogen was formed by stirring the aqueous suspensions in the

dark. Moreover, water alone did not show any evidence of H2 evolution upon the irradiation. However, the irradiation of TiO2 or Pt/TiO2 suspensions containing methanol led to the observable evolution of H2 gas. Fig. 5 illustrates the time course of photocatalytic H2 evolution from the Pt/TiO2 photocatalysts prepared by the SSSG, IWI, and PCD methods as well as from the Pt/P-25 prepared by the PCD method. Comparatively small H2 evolved was observed when unloaded TiO2 was utilized. The deposition of Pt on TiO2 resulted in a substantial improvement in the H2 evolution. The amount of H2 produced increased almost proportionally to the irradiation time in the investigated period. However, a slight decrease in the H2 evolution rate at long irradiation period was detected. This can be explained that in photocatalytic batch systems, long irradiation periods regularly cause a progressive decline in the H2 generation rate due to back reactions of products generated in the reaction system as well as pressure buildup in the gas phase [27]. As seen in Fig. 5, the H2 evolution from all Pt/TiO2 photocatalysts are in the same order of magnitude, nonetheless

T. Sreethawong, S. Yoshikawa / International Journal of Hydrogen Energy 31 (2006) 786 – 796

Fig. 4. TEM images of (a) 0.6 wt% Pt/TiO2 prepared by SSSG method, (b) 0.4 wt% Pt/TiO2 prepared by IWI method, and (c) 0.5 wt% Pt/TiO2 prepared by PCD method (Arrows: Indication of Pt nanoparticles).

the differences in the evolution rate can be clearly observed. For better clarity, the H2 evolution rate is represented as a function of Pt loading as shown in Fig. 6. It is manifest that

793

optimum amount of Pt loading for each preparation method exists. Irrespective of preparation methods, with the initial increase in the Pt loading amount, TiO2 particles may carry more Pt nanoclusters, which are indispensable for the removal of photogenerated electrons from TiO2 for the reduction reaction and lead to an increase in the photocatalytic activity. After reaching a maximum, the decrease in the activity with further increase in the Pt content beyond a certain optimum value may be, at least partly, due to too much Pt nanoclusters on TiO2 . These clusters would shield the photosensitive TiO2 surface, scatter the UV light to decrease the absorption by TiO2 , and subsequently diminish the surface concentration of the electrons and holes available for the reactions. Another explanation is that at high metal loadings, the deposited metal particles may act as the recombination centers for the photoinduced species [28]. Even though the existence of optimum Pt loading amounts was observed, such the amounts somewhat varied depending upon the preparation methods. As shown in Fig. 6, the optimum Pt loading amounts for SSSG, IWI, and PCD methods are 0.6, 0.4, and 0.5 wt%, exhibiting the H2 evolution rate of 1385, 1060, and 1210
mol h−1 , respectively. As mentioned earlier that the loaded Pt particles in the SSSG method may deposit on the surface of TiO2 through the mesoporous network instead of the outer and near the outer surface of TiO2 aggregates, the light shielding effect becomes less pronounced than the IWI and PCD methods, leading to the highest observed optimum amount of Pt loading. Additionally, some aggregations of Pt particles in the PCD method may be ascribed to a slightly higher value of the optimum Pt content than the IWI method. For the Pt/P-25 case, the highest photocatalytic activity could be observed at the rate of 583
mol h−1 over 0.4 wt% Pt/P-25. Besides, the activity of the Pt/P-25 over the entire range of Pt loading was much lower than that over the synthesized mesoporous Pt/TiO2 . There are many plausible reasons involving with the observation of much less photocatalytic activity over the Pt/P-25 photocatalysts: (1) the less active surface area (surface active sites) and the absence of mesoporous structure probably leading to less Pt distribution, (2) the larger crystallite size leading to more bulk photoinduced charge recombination before reaching the surface active sites, and (3) the presence of a portion of rutile phase, which possesses a lower flat band potential existing at almost similar level to the NHE potential (H+ /H2 level) than anatase phase, leading to less driving force for water reduction [29]. A particularly successful point to be underscored in this work is that over the investigated range of Pt loading amount over the synthesized mesoporous TiO2 , the SSSG Pt/TiO2 provided the highest photocatalytic activity, followed by the PCD and IWI Pt/TiO2 , respectively, as illustrated in Fig. 6. The interaction between excited species (TiO2 ) and trapping species (Pt) can be considered as a key mechanism in this reaction system because it signifies the capability of transferring the photoexcited electrons from conduction band of TiO2 to the Pt active sites and initiating the photocatalytic

H2 evolved / µmol

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H2 evolved / µmol

794

(a)

H2 evolved / µmol

Irradiation time / h

H2 evolved / µmol

Irradiation time / h

(c)

(d)

Irradiation time / h

Irradiation time / h

Fig. 5. Time course of photocatalytic H2 evolution over (a) Pt/TiO2 prepared by SSSG method, (b) Pt/TiO2 prepared by IWI method, (c) Pt/TiO2 prepared by PCD method, and (d) Pt/P-25 (Photocatalyst, 0.2 g; distilled water, 200 ml; methanol, 20 ml).

1500

H2 evolved / µmolh-1

1200 900 600 300 SSSG PCD

IWI P-25 PCD

0 0

0.2 0.4 0.6 0.8 Amount of Pt loaded / wt%

1

Fig. 6. Dependence of photocatalytic activity of H2 evolution on Pt loading content over photocatalysts prepared by three methods in comparison with P-25 support.

reaction. Therefore, the enhancement of the photocatalytic activity for H2 evolution over the largest surface area SSSG photocatalyst may be due to at least 2 possibilities: (1) the increased accessibility of reactant, water molecules, to the photocatalyst surface active sites existing along the mesopore network and (2) the high dispersion of Pt particles deposited through the mesopore structure of TiO2 support, leading to the better electronic interaction between the cocatalyst and support in the present system. Previous works also demonstrated the good interaction between cocatalyst and support in the sol–gel process [30,31]. Despite expected less such the interaction than the SSSG method, it convincingly becomes more considerable in the case of the PCD method than the IWI method, as pointed out by Karakitsou and Verykios [27]. For the PCD method, some aggregations of Pt particles, which may be useless, can affect the lower photocatalytic activity than the SSSG method as well. Another noticeable observation is the photocatalytic activity window of the Pt/TiO2 prepared from different methods.

T. Sreethawong, S. Yoshikawa / International Journal of Hydrogen Energy 31 (2006) 786 – 796

1800 At 0.6 wt% Pt loading

H2 evolved / µmolh-1

1500 1200 900 600 SSSG IWI PCD

300 0

50

(a)

Support

70 90 BET surface area / m2g-1

110

1800 SSSG IWI PCD

H2 evolved / µmolh-1

1500 1200 900

subsequent photocatalytic efficiency than the other two methods, 0.6 wt% Pt/TiO2 with surface area of 89 m2 g−1 provided the highest photocatalytic activity as earlier described to be the optimum point for Pt loading amount. Furthermore, as shown in Fig. 7(b) for the effect of crystallite size, the tendency reveals that the photocatalytic activity decreased as the TiO2 crystallite size increased. This is most likely due to the more preferential bulk recombination of the photogenerated charges prior to the trapping processes at the photocatalyst surface when the crystallite size becomes larger. However, it might be considered as a less pronounced variable owing to the insignificant change in the crystallite size of the photocatalysts with various Pt loading amounts among different preparation methods. Overall, the utilization of mesoporous TiO2 as the support for platinum was proved to be an effective means contributing to the considerably high photocatalytic activity of H2 evolution. In consequence of the fact that the observable discrepancies in the photocatalytic activity markedly depend on the preparation method, the Pt loading by SSSG method helps improve the physical characteristics directed to the enhancement of the photocatalytic functioning. This would also suggest a great opportunity to design and select new highly active photocatalysts.

600

4. Conclusions 300 0

(b)

795

Support 10

11

12 13 Crystallite size / nm

14

Fig. 7. Dependence of photocatalytic activity of H2 evolution on (a) BET surface area and (b) crystallite size of synthesized photocatalysts.

From Fig. 6, it is clearly shown that the SSSG Pt/TiO2 provided the broadest photocatalytic activity window towards the Pt loading amount. This also verifies the superior performance in photocatalytic H2 evolution of the mesoporous Pt/TiO2 prepared by the SSSG method to the other two conventional methods. Besides, in order to acquire more insight information on the relationship between the photocatalytic activity and physical properties of the synthesized Pt/TiO2 , the photocatalytic H2 evolution activity was correlated to the BET surface area and crystallite size of the photocatalysts as depicted in Fig. 7. From Fig. 7(a), the maxima of photocatalytic activity were observed at particular surface areas of the photocatalysts prepared by each method. As the change in surface area is ensued from the varied Pt loading amount (the higher the Pt loading, the lower the surface area, as shown in Table 4), the balance between these two factors is very decisive. Focusing on the mesoporous SSSG Pt/TiO2 possessing the higher surface area and

The mesoporous TiO2 with uniform and narrow pore size distribution prepared by a surfactant-assisted templating sol–gel process was utilized to support 0.1–0.9 wt% Pt by three loading methods, namely single-step sol–gel (SSSG), incipient wetness impregnation (IWI), and photochemical deposition (PCD) methods. The resulting Pt/TiO2 photocatalysts were systematically evaluated their performance via the photocatalytic H2 evolution in comparison with Pt/P-25 prepared by PCD method. The SSSG photocatalyst provided the superior photocatalytic activity over the entire range of Pt loading amounts to the others. The broadest photocatalytic activity window towards the content of Pt loading was also observed with the mesoporous Pt/TiO2 prepared by SSSG method. It should be, therefore, remarked that the coupled SSSG process with mesopore-controlling surfactant has unveiled as a useful technique for the preparation of metalloaded mesoporous photocatalyst with high photocatalytic activity.

Acknowledgments This work was financially supported by the Grant-in-aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan, under the 21COE Program and the Nanotechnology Support Project. Grateful acknowledgments are forwarded to (1) Prof. S. Isoda and Prof. H. Kurata and (2) Prof. T. Yoko at Institute for Chemical

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Research, Kyoto University for their continuous support of the use of TEM and XRD apparatus, respectively, and Dr. Y. Suzuki at Institute of Advanced Energy, Kyoto University for valuble suggestions.

References [1] Trong On D, Desplantier-Giscard D, Danumah C, Kaliaguine S. Perspectives in catalytic applications of mesostructured materials. Appl Catal A: Gen 2001;222(1–2):299–357. [2] Kresge CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS. Ordered mesoporous molecular-sieves synthesized by a liquidcrystal template mechanism. Nature 1992;359(6397):710–2. [3] Sayari A, Lui P. Non-silica periodic mesostructured materials: recent progress. Micropor Mater 1997;12(4–6):149–77. [4] Ying JY, Mehnert CP, Wong MS. Synthesis and applications of supramolecular-templated mesoporous materials. Angew Chem Int Ed 1999;38(1/2):56–77. [5] Yu-de W, Chun-lai M, Xiao-dan S, Heng-de L. Synthesis and characterization of mesoporous TiO2 with wormholelike framework structure. Appl Catal A: Gen 2003;246(1): 161–70. [6] Velu S, Kapoor MP, Inagaki S, Suzuki K. Vapor phase hydrogenation of phenol over palladium supported on mesoporous CeO2 and ZrO2 . Appl Catal A: Gen 2003;245(2): 317–31. [7] Linsebigler AL, Lu G, Yates Jr JT. Photocatalysis on TiO2 surfaces: principles, and selected results. Chem Rev 1995;95(3):735–58. [8] Mills A, Le Hunte S. An overview of semiconductor photocatalysis. J Photochem Photobiol A: Chem 1997;108(1):1–35. [9] Hoffmann MR, Martin ST, Choi W, Bahnemann DW. Environmental applications of semiconductor photocatalysis. Chem Rev 1995;95(1):69–96. [10] Cocco G, Campostrini R, Cabras MA, Carturan G. Propene hydrogenation on low-temperature reduced Pt/TiO2 . Effects of TiO2 phases and H2 treatment on specific catalytic activity. J Mol Catal 1994;94(3):299–310. [11] Yang JC, Kim YC, Shul YG, Shin CH, Lee TK. Characterization of photoreduced Pt/TiO2 and decomposition of dichloroacetic acid over photoreduced Pt/TiO2 catalysts. Appl Surf Sci 1997;121/122:525–9. [12] Falconer JL, Magrini-Bair KA. Photocatalytic and thermal catalytic oxidation of acetaldehyde on Pt/TiO2 . J Catal 1998;179(1):171–8. [13] Li Y, Lu G, Li S. Photocatalytic hydrogen generation and decomposition of oxalic acid over platinized TiO2 . Appl Catal A: Gen 2001;214(2):179–85. [14] Wang X, Yu JC, Yip HY, Wu L, Wong PK, Lai SY. A mesoporous Pt/TiO2 nanoarchitecture with catalytic and photocatalytic functions. Chem Eur J 2005;11(10):2997–3004. [15] Kawai T, Sakata T. Photocatalytic hydrogen production from liquid methanol and water. J Chem Soc Chem Commun 1980;15:694–5.

[16] Bamwenda GR, Tsubota S, Nakamura T, Haruta M. Photoassisted hydrogen production from a water-ethanol solution: a comparison of activities of Au–TiO2 and Pt–TiO2 . J Photochem Photobiol A: Chem 1995;89(2):177–89. [17] Lee K, Nam WS, Han GY. Photocatalytic water-splitting in alkaline solution using redox mediator. 1:Parameter study. Int J Hydrogen Energy 2004;29(13):1343–7. [18] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37–8. [19] Duff DG, Mallat T, Schneider M, Baiker A. Catalysts derived form polymer-stabilised colloidal platinum: effects of support and calcination on the catalytic behaviour in hydrogenation. Appl Catal A: Gen 1995;133(1):133–48. [20] Sreethawong T, Suzuki Y, Yoshikawa S. Photocatalytic evolution of hydrogen over nanocrystalline mesoporous titania prepared by surfactant-assisted templating sol–gel process. Catal Commun 2005;6(2):119–24. [21] Sreethawong T, Suzuki Y, Yoshikawa S. Synthesis, characterization, and photocatalytic activity for hydrogen evolution of nanocrystalline mesoporous titania prepared by surfactant-assisted templating sol–gel process. J Solid State Chem 2005;178(1):329–38. [22] Sreethawong T, Suzuki Y, Yoshikawa S. Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol–gel process with surfactant template. Int J Hydrogen Energy 2005;30(10): 1053–62. [23] Hague DC, Mayo MJ. Controlling crystallinity during processing of nanocrystalline titania. J Am Ceram Soc 1994; 77(7):1957–60. [24] Spurr RA, Myers H. Quantitative analysis of anataserutile mixtures with an X-ray diffractometer. Anal Chem 1957;29(5):760–2. [25] Cullity BD. Elements of X-ray diffraction. Addison-Wesley Publication Company: Reading, Massachusetts; 1978. [26] Rouquerol F, Rouquerol J, Sing K. Adsorption by powders and porous solids: principles, methodology and applications. Academic Press: San Diego; 1999. [27] Karakitsou KE, Verykios XE. Influence of catalyst parameters and operational variables on the photocatalytic cleavage of water. J Catal 1992;134(2):629–43. [28] Courbon H, Herrmann JM, Pichat P. Effect of platinum deposits on oxygen-adsorption and oxygen isotope exchange over variously pretreated, ultraviolet-illuminated powder TiO2 . J Phys Chem 1984;88(22):5210–4. [29] Karakitsou KE, Verykios XE. Effects of altervalent cation doping of TiO2 on its performance as a photocatalyst for water cleavage. J Phys Chem 1993;97(6):1184–9. [30] Bokhimi X, Morales A, Novaro O, López T, Chimal O, Asomoza M, Gómez R. Effect of copper precursor on the stabilization of titania phases, and the optical properties of Cu/TiO2 prepared with the sol–gel technique. Chem Mater 1997;9(11):2616–20. [31] Tseng IH, Chang WC, Wu JCS. Photoreduction of CO2 using sol–gel derived titania and titania-supported copper catalysts. Appl Catal B: Environ 2002;37(1):37–48.